Chemotherapy/Radiotherapy:
In the field of oncology, the detection, identification and characterization of cancer cells is an important aspect of diagnosis. Of the many challenges of medicine, none has had a more controversial beginning or has experienced more hard-fought progress than the treatment and cure of cancer. Effective treatment for most patients needed to reach every organ in the body to pin down the metastatic disease. More than 70% of cancer patients undergo chemo/radio therapy.
Despite the path breaking progress in oncology therapy from multiple angles, cancer cure still remains elusive. Advanced solid malignancies remain therapeutic challenges despite maximal therapy, in part, due to the development of resistance to radiation and chemotherapy. Eg Glioblastomas are among the most lethal of cancers with current therapies offering only palliation. Standard-of-care for glioblastoma consists of surgical resection, ionizing external beam irradiation, and chemotherapy. Radiotherapy has been the most effective nonsurgical treatment modality yet recurrence is essentially universal.
However, majority of patients undergoing chemo/radio therapy suffer from un-necessary severe side effects of the treatment. In addition many patients also show resistance to the treatment resulting in treatment failure.
Thus, there exists a need to develop diagnostic tests that can determine a priori the effectiveness of the prescribed chemo/radio therapy. Today, decisions for treatment are made on the basis of clinical parameters such as tumor histology, tumor volume as well as tumor stage and increasingly biological imaging techniques. Radiation/chemotherapy doses and schedules as well as combinations with drugs are prescribed as empirical class solutions under consideration of the tolerance limits of surrounding normal tissues. Since the current standard treatments prescriptions do not take in to account the heterogeneity/individuality of a tumor the therapies fail often and the patient undergoes un-necessary side effects.
Tumors typically have two kinds of cells, CSCs and tumor cells. CSCs constitute only a part of tumors and usually in minority. The bulk of tumor is made up of tumor cells which constantly divide and make the solid big mass called as ‘tumor’.
On the other hand CSCs are quiescent cells which hardly divide and have the ability to self-renew, i.e. divide in such a way that they make a daughter cell which is a perfect copy of them-selves plus make a cells which can go and differentiate into various cells of a particular tumor (Ref: The Biology of Cancer Stem Cells, Neethan A. Lobo et al., Annu Rev. Cell Dev. Biol. 2007. 23:675-99 and The theoretical basis of cancer-stem-cell-based therapeutics of cancer: can it be put into practice?, Isidro Sanchez-Garcia et al., BioEssays 29:1269-1280)
Chemo/radiotherapies work towards curtailing tumor size by targeting and killing fast dividing cells. Since CSCs hardly divide they do not get killed by these therapies and survive to make a tumor again, which is called as ‘relapse’ of a cancer. (Ref: Chemotherapy and Cancer Stem Cells, Jeremy N. Rich et al., Cell Stem Cell 1, October 2007; Identification of Selective Inhibitors of Cancer Stem Cells by High-Throughput Screening, Piyush B. Gupta et al., Cell 138, 1-15, Aug. 21, 2009; Identification and targeting of cancer stem cells, Tobias Schatton et al., BioEssays 31:1038-1049; TUMOUR STEM CELLS AND DRUG RESISTANCE, Michael Dean et al., Nature Reviews, cancer, Volume 5, April 2005; Cancer stem cells in solid tumors, Patrick C. Hermann et al., Seminars in Cancer Biology (2008)).
From the above references it can be seen that the CSCs have been isolated from fresh tumors based on certain cell surface markers that they have, using FACS (Fluorescent Activated Cell Sorting). These isolated CSCs as few as 100-200 cells were sufficient to initiate a new tumor as against up to 10000-20000 non CSCs/bulk tumor cells. (Ref: Prospective identification of tumorigenic breast cancer cells, Muhammad Al-Hajj et al., PNAS, Apr. 1, 2003, vol. 100_no. 7—3983-3988; Identification of a Cancer Stem Cell in Human Brain Tumors, Sheila K. Singh et al., CANCER RESEARCH 63, 5821-5828, Sep. 15, 2003; Identification of human brain tumour initiating cells, Sheila K. Singh et al., NATURE | VOL 432 | 18 Nov. 2004). CSCs were identified in mid 90s in blood cancers and first time in solid tumors in 2003 from breast cancers, followed by brain and all other cancers)
Presence of CSCs and lack of drugs directed against them could be one reason cancer cure has been eluding us. Multiple pharmas and biotechs are directing their efforts to invent drugs which will specifically target these CSCs in a tumor to prevent cancer relapse etc. Many of these drugs are in clinical trials.
The resistance to chemo/radio therapy and the treatment failure is due to presence of what are called Cancer Stem Cells (CSC) in the tumor. Tumors are heterogeneous in nature and contain 2 kinds of cells, cancer stem cells and tumor cells which form the bulk of the tumor. While it has been recognized for a long time that not all tumor cells have the potential to initiate a new tumor, or a recurrence after treatment, only recently methodological advances have emerged that eventually allowed identification of CSCs and to investigate their biology. CSCs have been prospectively isolated from a growing number of human cancers, including leukemias and tumors of the breast, brain, colon, head & neck and pancreas. For different tumors it has been shown that transplantation of CSC subpopulations led to higher tumor take rates when compared to unsorted populations from the same tumor.
A CSC is defined as a cell within the tumor that possesses the capacity to self-renew and to generate the heterogeneous lineages of all cancer cells that comprise a tumor. This implies that CSCs are possibly a small subpopulation of tumor cells which are able to expand the CSC pool or differentiate into cancer progenitor cells by symmetric or asymmetric division. However, this point is tumor dependent and in melanomas CSCs contributed to a significantly higher percentage of total tumor cells. The non-stem cells constitute the bulk of all cancer cells within the tumor, but have only limited proliferative potential and are non-tumorigenic.
Properties of CSCs:
CSCs are quiescent cells present in the tumor and thus are functionally different from the rapidly proliferating tumor cells which make up the bulk of the tumor. CSCs by virtue of being quiescent in nature are able to resist the ‘desirable effects’ chemo/radio therapy well as these therapies target the rapidly dividing cells of the tumor. In addition CSCs are endowed with multiple ion channels/transports, higher hypoxia tolerance and differential gene expression, etc. which contribute to their chemo/radio resistance phenomenon. An increasing body of data suggests biological differences of CSCs and non-CSCs are crucial to respond to the standard therapies and most pharmas are using these differences to design rational drugs against CSCs. For failure of radiotherapy and chemotherapy treatments, one underlying reason might be a low efficacy of current treatments on eradication of cancer stem cells (CSCs). Growing evidence indicating that CSCs are resistant to cytotoxic/radiation therapies and may thus contribute to treatment failure.
The current technologies that exist in the field similar to the instant disclosure include Oncotype Dx, and Mammaprint. These are similar but they do not detect presence of CSCs. They assess presence of ER/PR and Her-2-neu pathways in patients to assess if a patient needs post-operative chemotherapy. Currently there are no diagnostic tests which detect presence of CSCs in tumors and hence cannot predict relapse time and usefulness of chemo and radiotherapy. In addition, current methods do not offer help in choosing a particular chemotherapy drug/combination.
From a clinical point of view, the direct consequence of this concept is that cancer therapy can cure a patient only if all CSCs are eliminated and that a single surviving CSC can cause a recurrence or metastasis. In addition it also implies that if the tumor is assessed for presence of CSCs before prescribing the chemo/radio therapy there is a fair chance that the oncologist can predict the effectiveness of the treatment, manage the cancer treatment better and reduce the unwanted side effects of the treatment to patients in cases when the therapy is ineffective. Since the discovery of first solid tumor CSCs in 2004 there have been great advances in CSC biology. Predictive tests for content, distribution and sensitivity of CSCs, microenvironmental CSC niches and signatures should be used to allow CSC-based individualized tailoring of therapy within the class solutions.